Question: On the first 3 tests of her physics class, Jessica got an average score of 87. What does she need on the next test to have an overall average of 90?
Solution: Let her score on the next test be $x$ The sum of all of her scores is then $3 \cdot 87 + x$ The same sum must also be equal to $4 \cdot 90$ Solve: $x = 4 \cdot 90 - 3 \cdot 87 = 99$.